Biochemical studies have demonstrated that lentiviral RTs are highly error prone DNA polymerases, and their infidelity is a presumptive source of viral genomic hypervariability which allows viral escape from host anti-viral immune pressure. The hypothesis that we test in this proposal is that if fidelity of viral RTs is a genetic determinant of viral diversity essential for viral escape, then RT fidelity may also change during the course of viral infection. We have found that one simian immunodeficiency virus (SIV) RT allele, obtained from a pig- tailed macaque during the late symptomatic phase of infection (SIVMNE170), exhibits a greatly elevated level of fidelity, when compared to a RT allele isolated from the same animal at the initial infection stage (SIVMNECL8). This finding supports our hypothesis that viral RT fidelity may change in response to alterations in external selective pressure (i.e. changes in host immune function). Recently, we also found that RTs of SIVMNE35 and 84 from asymptomatic phase of infection has low fidelity, like the CL8 RT. This further supports our prediction that RT fidelity remains low when the host has relatively intact immune capability. Indeed, this 170 RT is the first in vivo high fidelity (hi-fi) RT isolated from natural course of viral infection. We also identified RT variants containing mutations that decrease fidelity at the early infection phase, supporting our hypothesis that RT fidelity changes over time. In this proposed work, we will delineate the evolutionary relationship between RT infidelity, host anti-viral immune capability and disease progression. First, we will systematically determine the fidelity of RT derivatives obtained from representative clones of SIVMNECL8 and SIVmac239 strains isolated at four different stages of viral infection. Secondly, we will use the in vivo altered fidelity RT mutants to understand novel structural and mechanistic determinants of RT infidelity. These proposed studies will enhance our knowledge of the genetic role of RT in the evolution, host immune selection and pathogenesis of immunodeficiency viruses.